The precious metal-rich, South Hercules mineralization, western Tasmania; a possible subsea-floor replacement volcanic-hosted massive sulfide deposit

Abstract

South Hercules is a newly delineated, disseminated to semimassive base metal sulfide deposit with significant gold and silver grades. The deposit lies a kilometer along strike south from the Hercules mine, a 2.0-Mt, polymetallic massive sulfide deposit in the Mount Read Volcanics belt, western Tasmania. The mineralization and alteration at the South Hercules deposit has been divided into two major facies: the mineralized sulfide facies and the carbonate facies. The carbonate facies interfingers with and surrounds the upper part of the sulfide facies. The mineralized sulfide facies includes three zones: a massive pyrite + or - barite zone, a siliceous + or - stringer sulfide zone, and a sphalerite-galena + or - pyrite zone. The carbonate facies also consists of three zones: a blebby carbonate-chlorite zone, a massive carbonate zone, and a cherty carbonate zone.The massive pyrite + or - barite zone contains significant gold grades and occurs mostly at the top of the sulfide facies. This zone is characterized by colloform aggregates of pyrite and compact massive pyrite in chert. The siliceous + or - stringer sulfide zone is a silicified and sericitized, tuffaceous shale with disseminated to stringer Pb-Zn mineralization. The massive sphalerite-galena + or - pyrite zone contains subordinate gold and silver and occurs as thin lenses toward the base of the sulfide facies. Significant zinc and lead values occur throughout the mineralized sulfide facies, with the highest values occurring along stratigraphically controlled layers. Copper values are conspicuously low throughout the South Hercules deposit, rarely exceeding 0.5 percent. The carbonate facies is conspicuous by its lack of significant precious metal and base metal grades.Mineragraphic investigations indicate that electrum occurs in two general associations: with pyrite, and with other sulfides such as sphalerite, galena, and tetrahedrite. The electrum locked in the pyrite euhedra is generally fine grained (<5-40 mu m), whereas the electrum associated with other sulfides displays a range between 5 and 200 mu m. Microprobe analysis indicates that the fineness (1,000 Au/Au + Ag wt %) for gold in the deposit ranges from 427 to 965.The delta 18 O isotope composition of the carbonate varies from 9.8 to 16.7 per mil, and the delta 13 C values range from -3.5 to +0.6 per mil. The calculated isotopic composition of the ore fluid at 150 degrees and 250 degrees C is consistent with derivation from evolved seawater. Sulfur isotope analyses of selected samples from the mineralized facies give delta 34 S values of 8.2 to 14.1 per mil; which is in the same range as values from the Rosebery deposit and consistent with values of sulfur derived from Cambrian seawater.The form, texture, mineralogy, stable isotopes, and fluid inclusion data for the South Hercules deposit suggest that it was formed largely by a subsea-floor replacement process from relatively low-temperature (150 degrees -250 degrees C), near-neutral fluids. The very low copper content and relatively high (Au + Ag)/(Pb + Zn) ratio of the ores support a low-temperature environment in which gold was transported as the Au(HS) 2 (super -) complex and Pb and Zn were transported as chloride complexes.